See article, p. 40

  • The adenosine 2A receptor antagonist ciforadenant was well tolerated and showed activity in renal cell carcinoma.

  • High expression of adenosine gene-expression signature genes correlated with tumor regression.

  • This phase I clinical trial justifies further study of ciforadenant's efficacy and predictive biomarkers.

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Triggering adenosine 2A receptors (A2AR) on the cell surface can suppress the antitumor effects of various immune cells, making A2AR signaling a potentially useful immunotherapy target. Fong and colleagues conducted a phase I clinical trial of the small-molecule A2AR antagonist ciforadenant alone or in combination with the anti–PD-L1 therapy atezolizumab. Sixty-eight patients with advanced, treatment-refractory renal cell carcinoma (RCC) with a median number of prior treatments of three were enrolled in the trial, with 33 patients receiving ciforadenant monotherapy and 35 patients receiving ciforadenant plus atezolizumab. Ciforadenant was generally well tolerated, with fatigue, pruritis, and decreased appetite being the most common treatment-related adverse effects in both the monotherapy and combination-therapy groups. RECIST-defined partial responses occurred in 3% of patients (one of 33) and 11% of patients (four of 35) receiving monotherapy and combination therapy, respectively. Notably, the estimated overall survival after a 25-month follow-up period was more than 90% in the combination-therapy group and more than 69% after a 16-month follow-up period in the monotherapy group, in line with some prior findings in trials of immune-checkpoint inhibitors in solid tumors that survival benefits may not always be reflected in response rates. Tumor regression was associated with high levels of expression of adenosine gene-expression signature genes, a surrogate for adenosine exposure used because of the extremely short plasma half-life of adenosine. In summary, this first-in-human study establishes that further trials to evaluate ciforadenant's efficacy and define biomarkers predictive of response are warranted.

See article, p. 54

  • MRTX849 is a potent, selective KRASG12C inhibitor with antitumor efficacy in mouse models.

  • In a phase Ib clinical trial, MRTX849 elicited partial responses in two patients with KRASG12C-mutant tumors.

  • MRTX849-resistant KRASG12C-mutant tumors were susceptible to combination treatment with other anticancer agents.

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Inhibitors targeting KRASG12C, a variant commonly found in lung and colon adenocarcinoma as well as some other cancers, are currently in the drug-development pipeline. Hallin and colleagues found that a novel drug, MRTX849, potently and selectively inhibits KRASG12C in vitro, likely through covalent binding to KRAS that stabilizes its GDP-bound (inactive) state. In mice bearing KRASG12C-mutant pancreatic cancer xenografts, MRTX849 treatment led to a dose-dependent increase in covalently modified KRASG12C, inhibition of ERK phosphorylation, and antitumor efficacy. In mice harboring a variety of KRASG12C-mutant tumors, some derived from cell lines and others from patients, antitumor efficacy was seen in the majority of models, with some exhibiting complete tumor regression. These results are in concordance with some early findings from a phase Ib clinical trial of MRTX849, in which two patients harboring KRASG12C-mutant tumors with progressive disease following several prior treatments attained rapid partial responses upon treatment with MRTX849. Additional in vivo experiments provided insight into the mechanism of MRTX849's antitumor activity, demonstrating that treatment with the drug modulates multiple KRAS-mediatedoncogenic signaling pathways. In vitro screens implied that HER-family, SHP2, mTOR, and CDK4/6 inhibitors may exhibit synergy with MRTX849, prompting in vivo experiments that demonstrated increased antitumor efficacy following combined treatment with MRTX849 and these inhibitors in models that had exhibited little or no susceptibility to single-agent MRTX849. These findings imply that even patients with KRASG12C-mutant tumors resistant to MRTX849 alone may derive benefit from MRTX849 combined with other agents and indicate the promise of this targeted therapy.

See article, p. 72

  • In patients with ER+, HER2 breast cancer, loss of expression of RB or PTEN was noted at time of progression.

  • PTEN loss reduced nuclear localization of p27, resulting in CDK4/6 and CDK2 activation in PTEN-deficient cells.

  • PTEN loss also confers resistance to PI3Kα inhibitors, possibly causing cross-resistance to multiple agents.

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Although combination treatment with CDK4/6 inhibitors and antiestrogen therapies is often effective in estrogen receptor–positive (ER+) breast cancer, resistance may develop, and the mechanisms underlying this acquired resistance are not fully understood. In paired biopsies obtained pretreatment and post-progression from five patients with ER+, HER2 breast cancer treated with the CDK4/6 inhibitor ribociclib and the aromatase inhibitor letrozole, Costa, Wang, and colleagues noted loss of expression of RB, PTEN, or both at the time of progression in four of the five patients. Experiments using ER+ breast cancer cells in vitro and in mouse xenograft models provided further evidence that loss of PTEN reduced susceptibility to multiple CDK4/6 inhibitors, and loss of RB conferred complete resistance. Notably, AKT inhibition in PTEN-deficient cells restored the efficacy of CDK4/6 inhibitors. Mechanistically, loss of PTEN appeared to reduce the localization of p27 to the nucleus, and restoring p27 localization to the nucleus partially rescued the susceptibility of PTEN-deficient cells to CDK4/6 inhibition. The reduction in nuclear localization of p27 in PTEN-deficient cells increased the activity of CDK4/6 as well as CDK2, allowing these cells to avert the blockade of the G1-to-S transition caused by CDK4/6 inhibitors in nonresistant cells. In summary, these findings demonstrate how loss of RB or PTEN can cause resistance to CDK4/6 inhibitors in ER+ breast cancer. Notably, loss of PTEN has also been observed to cause resistance to PI3Kα inhibitors; thus, this study shows how a single genetic event can cause clinical cross-resistance to multiple targeted therapies.

See article, p. 86

  • The axon-guiding protein SEMA4D allowed human breast cancer cells to cross the blood–brain barrier in mice.

  • SEMA4D alone was insufficient to establish brain metastases; MYC may promote tumor-cell survival in the brain.

  • This study provides insight into the little-understood molecular basis of brain metastasis.

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The molecular mechanisms that allow some circulating tumor cells (CTC) to cross the blood–brain barrier (BBB) and establish tumors in the brain are not well understood. Klotz and colleagues determined that CTCs derived from patients with luminal-type breast cancer generate metastases with similar tropisms when injected into mice, and CTC-derived metastatic cells isolated from different sites (brain, bone, lung, or ovary) exhibited different gene-expression profiles. Analysis of metastasis-free survival (MFS) data from 204 advanced primary breast tumors indicated that high SEMA4D expression in the primary tumor was associated with decreased brain MFS, but not lung or bone MFS, and CTC lines that exhibited brain tropism in mice had higher expression of SEMA4D, encoding an axon-guiding semaphorin protein. Additionally, in mice with subcutaneously implanted patient-derived breast-cancer xenografts, the only xenograft that caused spontaneous brain metastasis had the strongest SEMA4D staining. Further experiments implied that SEMA4D is required for CTCs to cross the BBB, but that other factors are required for colonization of the brain. One such factor may be MYC, one of the most upregulated proteins in CTC-derived brain metastases relative to metastases at other sites; indeed, deeper investigation showed that MYC may promote tumor-cell survival by reducing oxidative stress–induced apoptosis via upregulation of the antioxidant enzyme GPX1. Together, these results support a model in which SEMA4D and MYC act together to increase the risk of brain metastasis, with SEMA4D promoting extravasation of CTCs through the BBB, and MYC aiding those cells in establishing residence within the brain.

See article, p. 104

  • KRASG12R, unique to pancreatic ductal adenocarcinoma (PDAC), does not regulate macropinocytosis.

  • Underlying the defect in macropinocytosis, KRASG12R is defective in binding PI3K's catalytic subunit, p110α.

  • KRASG12R-mutant PDAC cells depend on p110γ upregulation for macropinocytosis.

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KRASG12R is present in 20% of pancreatic ductal adenocarcinomas (PDAC), but only about 1% of lung and colorectal cancers, which often harbor other KRASG12 missense mutations. Hobbs and colleagues found that KRASG12R-mutant PDAC cells, unlike other KRASG12-mutant PDAC cells, did not display a decrease in macropinocytosis—a nutrient-scavenging metabolic process thought to be essential for PDAC growth—in response to transient KRAS knockdown. Additionally, further experiments revealed that ectopic KRASG12R expression, unlike expression of other oncogenic KRASG12 mutants, did not increase macropinocytosis. X-ray crystallography showed that, in contrast to the better-studied oncoprotein KRASG12D, KRASG12R exhibits partial unfolding of switch II, a region important for interactions between KRAS and various effectors and regulators. Cells stably expressing KRASG12R displayed impaired signaling through the PI3K–AKT pathway as well as decreased phosphorylation of AKT and AKT substrates. These findings implied—and further experiments confirmed—that KRASG12R may be defective in binding PI3Kα's catalytic subunit, p110α, an AKT activator that promotes macropinocytosis. Instead of relying on p110α, KRASG12R-mutant PDAC cell lines depended on upregulation of p110γ for macropinocytosis. Experiments in PDAC organoids and a mouse model of PDAC based on patient-derived xenografts revealed that KRASG12R-mutant PDAC was more sensitive to the MEK inhibitor selumetinib than KRASG12D-mutant PDAC, and KRASG12R-mutant PDAC cell lines were sensitive to inhibition of ERK and autophagy. Collectively, these findings elucidate the effects of the KRASG12R mutation that distinguish it from other oncogenic KRAS mutations, providing a basis for further study of this unique variant.

See article, p. 124

  • Mucosal-associated invariant T (MAIT) cells promoted lung metastases in mice.

  • MAIT cell–mediated metastasis was dependent on tumor-expressed MR1 and suppression of lymphocyte function.

  • MR1-mediated MAIT-cell function may represent an interesting target for anticancer therapies.

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Mucosal-associated invariant T (MAIT) cells, which require MHC class I–related protein (MR1) to develop, are suspected to play a role in cancer, but this role is not well defined. In contrast to prior in vitro findings that MAIT cells exhibit antitumor effects, Yan and colleagues found that MAIT cells promote tumorigenesis, tumor growth, and metastasis in vivo. MR1−/− mice injected with melanoma cells exhibited reduced lung metastases compared with MR1–wild-type (MR1WT) mice, suggesting that lack of MAIT cells suppressed lung metastasis. Corroborating this interpretation, the number of lung metastases in MR1−/− mice into which MAIT cells had been adoptively transferred was similar to the number in MR1WT mice. Further experiments established that upregulation of MR1 on the surfaces of tumor cells able to present MAIT-activating ligands activated MAIT cells and implied that it is a tumor MR1–MAIT cell interaction, rather than a host MR1–MAIT cell interaction, that resulted in increased lung metastases. MAIT cells appeared to promote metastasis through suppression of NK-cell effector function via MAIT cell–derived IL17A. Ablation of surface expression of MR1 on tumor cells reduced their metastatic potential, and antibody-mediated MR1 blockade suppressed lung metastasis as well as subcutaneous tumor growth, possibly by increasing tumor infiltration by and effector function of tumor-infiltrating lymphocytes. Collectively, these results provide a new paradigm for understanding the tumorigenic effects of MAIT cells in cancer, suggesting that MAIT cells may promote cancer in vivo and that targeting MAIT cell function may be useful in anticancer therapies.

See article, p. 142

  • Half of pancreatic ductal adenocarcinomas (PDAC) retain normal TGFβ-pathway function.

  • Dysregulated expression of ID1 can suppress apoptosis in PDACs with an intact TGFβ pathway.

  • Investigation of identified small-molecule inhibitors of ID proteins may be of interest in PDAC.

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Preventing apoptosis of premalignant cells, a process mediated by the tumor suppressor TGFβ, plays an important role in the development of pancreatic ductal adenocarcinoma (PDAC); however, only about half of PDACs exhibit genetic inactivation of the TGFβ pathway. Huang and colleagues found evidence that dysregulated expression of inhibitor of differentiation 1 (ID1), a protein that normally inhibits progenitor-cell differentiation, explains how PDAC cells with normal TGFβ-pathway function avoid undergoing apoptosis. A large proportion of PDAC cells—including progenitor cells—exhibited ID1 expression, and downregulation of ID1 was associated with increased apoptosis in PDAC cells. In PDAC cells in which the TGFβ pathway remained active, TGFβ did not cause the downregulation of ID1 expression seen in normal cells. Further experiments suggested that aberrant sustained ID1 expression leads to an uncoupling of the TGFβ-mediated epithelial–mesenchymal transition from apoptosis. ID1 expression could be induced by genetic alterations that activate PI3K–AKT signaling, which are observed in a subset of patients with PDAC whose tumors do not harbor inactivating mutations in the TGFβ pathway. In summary, this study identifies ID1 as a critical PDAC-promoting factor in tumors in which the function of the TGFβ pathway is intact. Notably, small-molecule inhibitors of ID proteins have been discovered, and this work suggests that investigation of these inhibitors in PDAC may be of interest.

Note:In This Issue is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details.